The present invention relates to a process for dimensionally stabilizing flexible foam polyolefin articles utilizing a nitrogen-containing compound having an unsaturated hydrocarbon moiety.
Cellular plastics have many important uses. Today flexible foam articles made from polyolefins have well known applications in the packaging, construction, sports, appliance, and automotive markets. Important qualities of flexible foams and manufacturing processes therefor, include low shrinkage, dimensional stability, and cost of manufacture. Expanded (foamed) articles, as the term is used herein, are cellular polymers having a two-phase gas-solid system, in which the solid is continuous and comprises a synthetic polymer. The gas phase is usually distributed in cells within the article.
It is well known to make closed-cell olefinic polymer resin foams by the process of extrusion foaming wherein a normally solid thermoplastic olefinic polymer resin, such as polyethylene, is heat-plastified and mixed under pressure with a volatile material, such as 1,2-dichlorotetrafluoroethane (FC-114) to form a flowable gel which is then passed through a shaping orifice or die opening into a zone of lower pressure. Upon the release of pressure, the volatile constituent of the gel vaporizes, forming a gas-phase cellular structure in the gel which cools to a corresponding cellular foamed solid resin. Desirably, the resulting gas cells are substantially uniform in size, uniformly distributed through the foam body, and closed, i.e., separated from each other by membrane walls of resin. Although a number of general principles are thought to be understood, much of the extrusion foaming technology is empirical.
One of the common requirements of acceptable foam resin products is dimensional stability, i.e., it is desired that the linear dimension and volume of a piece of foam resin not change appreciably, either to shrink or to expand, under ordinary conditions, from the time its manufacture is complete until the time its ultimate useful life is ended. It is also desired that if any appreciable shrinking of a foam is to occur, which is usually the case with a freshly extruded foam, the foam be able to recover within a reasonable period of time to a substantially constant volume close to that of the foam measured shortly after its extrusion.
With respect to dimensional stability, it is noted that the vapors of volatile material originally present in foam cells gradually permeate the cell wall and escape from the foam over a period of time, thereby tending to reduce the inner cell pressure and tending to cause the foam to shrink during that time. However, when the foam is exposed to ambient atmosphere, air and its constituent gases also tend to permeate into the foam through the cell wall over a period of time, thereby increasing the inner cell pressure. By virtue of the possibly differing relative rates of permeation of the blowing agent out of and air into the cellular structure, there exists the potential for the development of a significant pressure differential (relative to ambient air pressure) within such cellular structure with attendant shrinking or expansion thereof during the indicated air/blowing agent interchange therein. Accordingly, the difficulties of attaining dimensional stability present a problem frequently encountered which is particularly acute in foams of relatively low density (high expansion ratio) when the resin membrane cell walls are relatively thin.
Many additives have been proposed which ameliorate the dimensional stability problem in various compositions and processes.
U.S. Pat. No. 4,214,054 (Watanabe et al) describes expanded polymer articles and a process for making same from synthetic resins such as polyethylene or ethylene-vinyl acetate copolymer resin utilizing 0.1 to 10 weight percent of an additive comprising saturated higher fatty acid amides, saturated higher aliphatic amines and complete esters of saturated higher fatty acids. The expanded articles so-made are described as having little shrinkage after expansion.
U.S. Pat. No. 4,368,276 (Park) describes the use of certain N-substituted fatty acid amide compounds of formula A: ##STR2## wherein R.sub.1 represents an alkyl group of from 11 to 17 carbon atoms and R.sub.2 represents an alkyl group of from 12 to 18 carbon atoms. The described amides which have saturated branches include such compounds as N-stearyl stearamide, N-palmityl palmitamide, etc.
U.S. Pat. No. 4,345,041 (Hoki et al) describes the use of an N-higher aliphatic hydrocarbyl substituted amide of a C.sub.1 -C.sub.8 aliphatic carboxylic acid as a shrinkage control agent for olefin polymer foams. Suitable stabilizers are said to include amides of formula B: ##STR3## wherein R.sub.1 is an aliphatic hydrocarbon group having 10 to 24 carbon atoms; R.sub.2 is a hydrogen atom, an aliphatic hydrocarbon group having 1 to 24 carbon atoms or a oxyalkylene or polyoxyalkylene group represented by (--R.sub.4 --O--).sub.n A wherein R.sub.4 is a residual group of an aliphatic polyhydric alcohol having 1 to 5 carbon atoms, A is a hydrogen atom or a residual group of an aliphatic carboxylic acid having 1 to 24 carbon atoms and n is an integer ranging from 1 to 20; and R.sub.3 is a hydrogen atom or a aliphatic hydrocarbon group having 1 to 7 carbon atoms.
U.S. Pat. No. 4,331,779 (Park) describes ethylenic polymer foams having improved dimensional stability which foams utilize (during the preparation thereof) a monoethylenically unsaturated carboxylic acid such as acrylic acid.
While the above compositions have been found useful in dimensionally stabilizing certain polymer foams, it is desirable to provide additional stabilizing additives for greater process operational flexibility.